The present invention relates generally to radars, and more particularly to imaging radars for producing radar images of objects that are located behind obscuring structures, such as concrete and stucco barriers, and the like.
The surveillance of areas which are restricted from optical view can be of great help to law enforcement and military personnel. Law enforcement officials have a need to identify and locate persons inside concrete and stucco buildings, and the like. Impulse radar has been highly touted for use in this application, but has had many problems, such as interference from television and other communications transmitters.
The present imaging radar is an adaptation of the concepts used in a three dimensional imaging radar developed by the assignee of the present invention, and described in U.S. patent application Ser. No. 08/028,451, filed Mar. 9, 1993 which is assigned to the assignee of the present invention. The three dimensional imaging radar system utilizes ultra wideband linear FM millimeter wave radar technology to provide high resolution target information. The three dimensional imaging radar provides continuous long term surveillance of remote areas with full video recording of all surveillance activities. The three dimensional imaging radar uses a dual band radar operating at 56 GHz and at 14 GHz. The three dimensional imaging radar demonstrated the capability to penetrate certain materials such as gypsum (wall) board, wood, and some forms of brick. However, the three dimensional imaging radar cannot see through concrete or stucco structures or barriers because the concrete material has excessive loss for any practical imaging of objects on the other side of the barrier.
More particularly, in an attempt to solve the problem of concrete penetration, laboratory tests were conducted using radars developed by the assignee of the present invention that operate in the frequency range of 500 MHz to 2 GHz. A linear frequency modulated continuous wave (FMCW) radar and an impulse radar were both tested. These low frequency (ultra wide band) radars demonstrated that acceptable attenuation of just a few db was experienced for common reinforced block wall, concrete prefab reinforced walls and. California stucco materials. These tests also demonstrated that a range resolution of approximately 6 inches could be achieved with portable equipment.
The decreased attenuation at the lower frequencies is believed to be related to the size of the "aggregate material" within the concrete relative to the wavelength of the radar energy. The typical size of the sand and rock in the concrete is greater than a quarter wavelength (0.050 inches) at the 56 GHz frequency. At 1 GHz, a quarter wavelength is 3 inches and the concrete appears to influence the wave much less.
One key limitation at these lower frequencies is antenna beam size. A radar operating at 56 GHz can achieve a 0.6 degree pencil beam (two way) which produces a 2 inch beam size at 16 feet. The 56 GHz radar antenna is a 1 foot diameter parabolic dish. A 14 GHz radar uses the same dish antenna and has a 1.5 degree two way beam width. To achieve a 0.6 degree beam at 1 GHz, an antenna with a diameter of 56 feet is required. This is quite impractical, therefore a different technique must be used to achieve acceptable angular resolution for radars operating at the lower frequencies.
Accordingly, it is an objective of the present invention to provide for imaging radar systems that permit imaging of targets located behind obscuring structures, such as concrete and stucco barriers, and the like. It is a further objective of the present invention to provide for imaging radar systems that provide three dimensional imaging of targets located behind obscuring structures.